The present invention is directed to an apparatus and method for indicating a difference between a first voltage input signal and a second voltage input signal, such as operational amplifiers for indicating differences between two voltages.
Analog circuits often have need for high-speed operational amplifiers (also known as op amps) that are capable of transmitting signals with little distortion caused by the op amp itself. There are known op amp architectures that are suitable for high-speed applications. Two common such high-speed op amp architectures are current feedback op amps and class AB op amps. These two representative op amp architectures use a common output stage that has been proven useful because of its wide bandwidth and low distortion performance.
Some output stages for op amps exhibit an exponential relationship between the output signals produced by those output stages and the voltage signals applied to an associated input stage. An example of an op amp output stage exhibiting such an exponential relationship is a type of output stage known as a dynamic diamond driver output stage. The exponential relationship means that the higher a differential voltage across the inputs of the op amp, the higher will be the current applied for charging and discharging a compensation capacitor at the high impedance node of the output stage. Such a relationship is desirable because it increases the slew rate of the op amp, which is manifested in the further benefit of decreasing distortion. Employing such a dynamic diamond driver output stage in an op amp provides benefits in increasing speed of the transistors on the signal path and provides higher driving capability. The speed increase is manifested as improved linearity and consequent lower distortion. Higher current driving capability eliminates distortion products due to driving limitations, such as turning off one of the pre-divers when heavy loads are required.
The dynamic diamond driver has a significant disadvantage of asymmetry with respect to the input signal. This occurs because the biasing current going into the pre-driver transistors is only going to increase when the signal is passing through them. While one signal path is very fast, the other signal path is very slow, and this disparity in speed causes distortion when the output signal changes from a negative value to a positive value.
Speed in the output stage pre-driver transistors of an op amp is important in circuits requiring high-speed, low distortion performance because those transistors are directly connected to the high impedance node of the output stage where most of the distortion is created. If one of the pre-drivers has less current during a signal transition the overall speed of the amplifier is decreased between signal transitions, thereby causing undesirable distortion products. These distortion products occur during signal transitions principally because once a bipolar transistor is near turning off, it takes an unacceptably long time to turn it back on again.
There is a need for an apparatus and method for indicating a difference between a first voltage signal and a second voltage signal, such as an op amp, that performs symmetrically during signal transitions.
An apparatus for indicating a difference between a first voltage and a second voltage includes: (a) an input unit for receiving the first voltage at a first locus and receiving the second voltage at a second locus; the input unit quanitifying the difference; (b) an output unit coupled with the input unit and cooperating with the input unit to generate an output signal for effecting the indicating; and (c) a signal treating unit coupled with the output unit, the first locus and the second locus, and employing at least one algorithmic relation with at least one of the first voltage and the second voltage to generate at least one bias current for effecting a substantially balanced response by the output section in generating the output signal as the difference varies. The at least one bias current has nonnegative values as the difference ranges in value.
A method for indicating a difference between a first voltage and a second voltage includes the steps of: (a) in no particular order: (1) providing an input unit for receiving the first voltage at a first locus and receiving the second voltage at a second locus; the input unit being configured for quanitifying the difference; (2) providing an output unit coupled with the input unit; the output unit being configured for cooperating with the input unit to generate an output signal; the output signal effecting the indicating; and (3) providing a signal treating unit coupled with the output unit, coupled with the first locus and coupled with the second locus; and (b) operating the signal treating unit employing at least one algorithmic relation with the first voltage and with the second voltage to generate at least one bias current for effecting a substantially balanced response by the output section in generating the output signal as the difference varies; the at least one bias current having nonnegative values as the difference ranges in value.
It is, therefore, an object of the present invention to provide an apparatus and method for indicating a difference between a first voltage signal and a second voltage signal that performs symmetrically during signal transitions.
Further objects and features of the present invention will be apparent from the following specification and claims when considered in connection with the accompanying drawings, in which like elements are labeled using like reference numerals in the various figures, illustrating the preferred embodiments of the invention.